Time of flight ranging systems are commonly used in level measurements, and are referred to as level measurement systems. Level measurement systems are used to determine the distance to a reflective surface (i.e. reflector) by measuring how long after transmission of a burst of energy pulses, an echo is received. Such systems may utilize ultrasonic energy or microwave energy.
Ultrasonic-based level measurement devices typically take the form as shown in FIG. 1 which comprises an ultrasonic transducer housing 10. The ultrasonic transducer housing 10 contains an ultrasonic transducer and the electronic circuitry for providing the level measurement functionality. The ultrasonic housing 10 includes a transducer face 12, typically on the lower or emitter surface, which allows for the transmission and reception of ultrasonic pulses from the transmitter contained in the housing. The ultrasonic level measurement device 10 is coupled through a cable to a computer or a control panel capable of handling multiple devices. The cable 14 provides signals for controlling operation of the transducer, i.e. generation of transmit pulses and reception of receive echo pulses. The receive echo pulses are processed by the control panel to determine the level measurement readings.
Microwave-based level measurement systems 20, unlike ultrasonic-based systems, have a microwave antenna 22 which is coupled to the exterior of the housing 24. The housing 24 contains the electronic circuitry and includes an interface for connecting to the microwave antenna 22. The microwave antenna 22 comprises a dielectric rod antenna as shown in FIG. 2. The rod antenna 22 is made from a low dielectric material such as PTFE. For a less intrusive design, for example for measurement applications in a shallow vessel, the microwave rod antenna 24 (FIG. 2) is replaced by a microwave horn antenna 24 as shown in FIG. 3. The horn antenna configuration is also less susceptible to problems resulting from build-up on the antenna, for example. In processes where there is splashing.
Microwave is affected less than ultrasonic by the gas or dust in the transmission path. In level measurement applications, microwave-based devices have the advantage of providing better resolutions and therefore more precision over ultrasonic-based devices. In addition, microwave-based devices are much more temperature insensitive than ultrasonic-based devices. Ultrasonic-based devices, on the other hand, have a more compact configuration and typically cost less and as such have found widespread use in many level measurement applications.
Even with the advantages of microwave-based systems the inherent incompatibility between the configuration of a microwave-based level measurement system and an ultrasonic-based level measurement system makes a retrofit essentially a new installation for the transducer. In addition, differences in the output signals and signal conditioning for microwave-based and ultrasonic-based level measurement devices do not allow for the interchangeability between the two types of devices in a control panel or level measurement system.
Accordingly, there remains a need for microwave-based level measurement systems which are essentially field interchangeable with ultrasonic-based level measurement or time of flight ranging systems.